How to have an object follow the surface of another object?
You can use a Particle System. Use your terrain as the emitter and the people as either an Object or a Group (Set their Origin to the bottom).

Set the Start and End time to 1.

Under Velocity change "Normal" to zero.



Choose either Object or Group (You'll need to separate your planes in Edit Mode press "P" and select Loose Parts). "Object" would be choosing one plane/person as the particle. With "Group" you could choose any number of planes/people for more variety (Select the planes in Object Mode and press CTRL +"G" to Group them).




Additionally you can have control over the Particle Systems density by Weight Painting and a Vertex Group. With your Emitter selected go into Weight Paint Mode and select an Add Brush (There also is a Subtract Brush).

 



Preform your Weight Painting. Blue represents the most density and red the least. Colors in between vary the density. This creates a Vertex Group with the default name "Group".



Go back to the Particle Emitter and under Vertex Groups > Density select or enter the name "Group".